Low density foamed polyhedron cell structures are well known. The conventional open-cell foam structure consists of a plurality of inter-connected, three dimensional cells which are generally convex. In a conventional open-cell structure, all or a portion of the cell faces may be absent, but the cells are intercommunicating and the cellular structure is retained. Depending upon the molecular structure of the material, a foamed cellular material may range from quite rigid to a material that is soft and flexible. The flexible foamed cellular structures are resilient and recover their original shape after deformation.
All known engineering materials including open-cell foam cellular structures have a positive Poisson's ratio and thus contract laterally when stretched and expand laterally when compressed. Also, bent beams of conventional materials which have a positive Poisson's ratio display the conventional cross-sectional configuration known as "anticlastic curvature".
There are known techniques for modifying the compress/deflection characteristics of certain types of open-cell foam materials. One of these techniques is described in U.S. Pat. No. 3,025,200 issued on Mar. 13, 1962 for an invention by William R. Powers entitled "Celliform Structure and Method of Making Same". This patents teaches that if a foam material is permanently compressed, its properties can be changed so that the material responds with linear strain when linear stress is applied. Conventional untreated materials produce non-linear response. However, the teaching of the foregoing patent is the application of compression in one direction only, and the resulting material has a positive Poisson's ratio.
If an open-cell foam material could be produced with the property of a negative Poisson's ratio, there would be numerous possible applications such as fasteners, gaskets and other seals, as well as applications for shock absorbing and cushioning materials. There is therefore a need for an improved material of the open-cell foam type having a negative Poisson's ratio. There is specially a need in many applications for such a material if such material could be produced by a simple and inexpensive method.